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The endodermal cells of the human yolk sac (YS) produce non-nucleated erythrocytes (NNEs) and numerous serum proteins that are transiently storage within the YS cavity. After their transfer via the vitelline duct to the embryo gastrointestinal lumen, the nutrients' final fate is unknown. With the aim of investigate how erythroid cells and nutrients are conveyed to embryo circulation, we studied, using a morphological and immunohistochemical approach, the embryo anatomy and the serum protein α-fetoprotein (AFP) presence, in 15 human embryos and their YS, collected from tubal pregnancies from 4 to 8 wpf. We observed at 5 wpf, a strong AFP staining in the endodermal cells of the YS, thereafter AFP was only present in the YS cavity and the gastrointestinal lumen. During 7 wpf, AFP expression declined and disappeared, concomitant with YS regression. Between 5 and 7 wpf, NNEs were observed in the gastrointestinal cavity, where they accumulate in the stomach. Here, the cells were attached to the endodermal epithelial cells or were free in the lumen. By scanning electron microscopy, we identified signs of NNEs phagocytized by endodermal cells. Those NNEs free in the lumen, after hemolysis, were probably removed by endocytosis (cell debris). Taking all together, we postulate that after reaching the endodermal epithelial cells of the stomach, nutrients are transferred to the embryo by a phagocytic/endocytic mechanism that is operative until the end of 6 wpf. After absorption, NNEs are probably degraded within phagosomes, nutrients delivered to the cell cytoplasm and then transported towards the embryonic circulation., (© 2015 Wiley Periodicals, Inc.)

During the early human embryonic period nutrients and blood cells are temporarily provided by the extraembryonic yolk sac (YS). The YS before week six is involved not only in primitive but also in definitive erythropoiesis. While the destiny of primitive erythroid cells that fill the blood vessels of the YS is well known, the final destination of erythrocytes present in the endodermal vesicular system is unknown. In the present study we have investigated, step by step, the destiny of the erythrocytes present in the endodermal vesicles during the embryonic period. Twelve human YSs and their corresponding yolk stalks were analyzed between weeks 4 and 7 of embryonic age by light and scanning electron microscopy. It is shown that erythrocytes (according to their size and morphological features) located within the endodermal vesicles of the YS wall are pulled out through endodermal pits into the YS cavity, from where they reach the lumen of the primitive gut of the embryo through the vitelline duct, a temporary pathway communicating both compartments. During the study period no erythrocytes were seen within the embryo's vascular network where only primitive erythroblasts were identified. Our results indicate that the vitelline duct plays an important transient role as a pathway for the transport of nutrients and blood cells between the YS and the embryo before week five of embryonic development that ends just at the time when YS-embryo circulation becomes established., ((c) 2010 Wiley-Liss, Inc.)

In this paper we present an in situ ultrastructural cytochemical study on the distribution and spatial-temporal expression of proteoglycans (PGs) in the extracellular matrix of the migratory pathway of mouse primordial germ cells (PGCs) during the different phases of migration, by the use of the cationic dye ruthenium hexammine trichloride (RHT). Embryos of 9, 10, 11 and 12 days of development were used. The treatment with RHT revealed PGs as electron dense layers, granules, and filaments. Whereas granules prevailed in the extracellular spaces of the migratory route during the whole migratory process, the amount of filamentous structures increased during the migration phase of PGCs. At the end of the migratory process the surface of the PGCs lost its reaction by RHT. There were differences in the size of the granules of PGs at the initial migratory period (9-day-old embryos) as compared with the other days of gestation. There was a strong reaction for PGs in the extracellular spaces, expressed as a meshwork of granules interconnected by filaments, as well as reaction on the basement membranes during the peak of the PGCs migration in 10-day-old embryos. These results support the hypothesis that these molecules may have an important role in the migration of PGCs, although the precise mechanism involved in this process is not yet clear.

Primordial germ cells (PGCs) were studied by electron microscopy in human embryos at 27 and 30 days of gestation. PGCs were mainly found in the hindgut epithelium, some at the stage of separation from the endoderm, and others in migration through the mesenchyme of the dorsal mesentery between the primitive intestinal wall and the coelomic epithelium, including the coelomic angle. A few PGCs were still located in the endodermal epithelium. Before the process of separation and migration, PGCs appeared clearly different from neighboring somatic cells (endodermal, mesenchymal and epithelial cells of the hindgut and the coelomic layer). PGCs located in the endodermal epithelium were large and showed an irregular cell body often provided with filopodia-like processes that contacted somatic cells to form small areas of focal junctions. As a rule, PGCs possessed a large, round nucleus containing one or two conspicuous nucleoli. Lipid droplets, abundant glycogen particles, ribosomes and mitochondria were often observed in their cytoplasm. A detailed ultrastructural analysis revealed a delicate fibrillar coat frequently present on the free surface of PGCs during their migratory phase through the mesenchyme of the dorsal mesentery. This surface coat, likely corresponding to a glycocalyx, appeared as a particular, conspicuous filamentous layer of 30 nm thickness, mainly in the leading pseudopodial projections of PGCs migrating toward their target. The surface coat of PGCs can be associated with the binding sites of specific macromolecular components of the extracellular matrix, including fibronectin, and hence plays a role in PGCs recognition and migration during this special embryonic phase.(ABSTRACT TRUNCATED AT 250 WORDS)

The ultrastructure of a 4-cell human embryo undergoing development in vivo is described. The embryo recovered from the Fallopian tube 130 h after a single episode of intercourse and 70 h after the luteinizing hormone peak in plasma, was partially surrounded by cumulus cells. The blastomeres, of even size, were nucleated and had a similar distribution of cytoplasmic organelles. No signs of abnormality or cellular degeneration were observed. Transmission electron microscopy of serial sections revealed the presence of primitive desmosomes between cells, abundant vesicles within the blebs of the outer sheet of the nuclear membrane, blebbing of the inner sheet of the nuclear membrane and close association between the inner sheet of the nuclear membrane and the intranuclear annulate lamellae. Nucleolar precursors lacking the structural organization of mature nucleoli were also found. Similarities and differences between this and other preimplantation human embryos reported earlier are analysed.